1. Field of the Invention
The present invention relates to a stacked semiconductor device and a method of manufacturing the same.
2. Description of the Related Art
To realize miniaturization and high-density packaging of the semiconductor device, a stacked semiconductor device having plural semiconductor elements stacked and sealed in one package has already been put into practical use. In the stacked semiconductor device, the plural semiconductor elements are sequentially stacked on a circuit substrate such as a wiring board and a lead frame via an adhesive layer. The electrode pads of the individual semiconductor element are electrically connected to the connection portions of the circuit substrate through metal wires. Such a laminated body is packaged with a sealing resin to configure the stacked semiconductor device.
The above-described stacked semiconductor device has a possibility that when the semiconductor elements having the same shape are stacked or a semiconductor element having a larger size is stacked on a lower and smaller semiconductor element, the metal wires connected to the lower semiconductor element are contacted to the upper semiconductor element. Therefore, it is important to prevent the occurrence of an insulation failure, a short circuit or the like due to the contact of the metal wires with the upper semiconductor element. Then, a bonding method using the metal wires is improved in various ways to control a loop height of the metal wires connected to the lower semiconductor element to a low level.
Known bonding methods capable of lowering the height of the wire loop include a connecting method (1) which forms a metal bump on an electrode pad of a semiconductor element, performs ball connection of one end of the metal wire to a connection portion of a circuit substrate and pressure-bonds the other end of the metal wire to the metal bump on the electrode pad (a reverse bonding method (see JP-A 2005-328005)), and a connecting method (2) which performs ball connection of one end of a metal wire to an electrode pad of a semiconductor element, squashes the top portion of the ball together with a part of the metal wire, lets out the metal wire, and pressure-bonds the other end of the metal wire to the connection portion of the circuit substrate (see JP-A 2004-172477).
The reverse bonding method (1) can decrease the loop height in comparison with the ordinary bonding (forward bonding), but the conventional method has the height of the metal bump as an obstacle to the further reduction of the height of the wire loop because of the pressure bonding of the metal wire to the metal bump formed on the electrode pad of the semiconductor element. According to the bonding method (2), even if the ball bonded to the electrode pad of the semiconductor element is squashed, the reduction of the height of the ball is limited, and it is difficult to make the loop height of the metal wire lower than, for example, the height of about two times of the wire diameter including the ball-bonded portion.
JP-A 2005-116916 discloses that a metal wire is connected to an electrode pad, which is formed at the vicinity of the center of a semiconductor element, by performing ball connection of one end of the metal wire to the electrode pad, the metal wire is let out in contact with an insulating film (silicon nitride film) of the surface of the semiconductor element so that the metal wire has an M-shaped loop (a loop shape having three bent portions), and the other end of the metal wire is connected to an external terminal by pressure bonding. The simple provision of the metal wire with the M-shaped loop can prevent the contact between the outer circumference of the semiconductor element, but cannot decrease sufficiently the loop height of the metal wire.